Prebreeding moult, plumage and evidence for a presupplemental moult in the Great Knot Calidris tenuirostris

We studied the prebreeding moult and resulting plumage in a long-distance migrant sandpiper (Scolopacidae), the Great Knot Calidris tenuirostris, on the non-breeding grounds (northwest Australia), on arrival at the staging grounds after the first migratory flight (eastern China) and on or near the Russian breeding grounds (Russian data from museum specimens). We show that breeding plumage scores and breast blackness were affected not only by the increase in moulted feathers but also in the wearing down of overlaying pale tips of fresh feathers. Birds migrating from Australia and arriving in China had completed or suspended moult, but more moult must occur in Asia as Russian specimens had moulted more of their mantle and scapular feathers. Russian birds had significantly more red feathering on their upperparts than had birds in Australia or those arriving in China. The increase in reddish feathers cannot by accounted for simply by continuation of the prealternate moult. Instead, a third, presupplemental moult must occur, in which red-marked feathers replace some scapular and especially mantle feathers that were acquired in a prealternate moult only 1-3 months earlier. Great Knot sexes show little size and plumage dimorphism, whereas two other sandpipers that have supplemental plumages (Ruff Philomachus pugnax and Bar-tailed Godwit Limosa lapponica) are thought to be highly sexually selected. Bidirectional sexual selection may therefore be involved in the evolution of a supplemental plumage in Great Knots.